How solar heating works!

Explore here how Solar heating Works, what you can do with it and all the technical stuff:

  1. How Solar Heating Works
  2. Can I fit a solar water heating system to my existing hot water tank?
  3. Which is best, flat panel or evacuated tube?
  4. What happens when I don’t need any more hot water and the sun is shining?
  5. Solar for Swimming Pools
  6. Advantages of the Radiant Solar system
  7. Technical Information on the Radiant Solar collector
How Solar Heating Works

Solar Thermal heating works by absorbing the warmth from the sun into a panel or collector. We have all felt that warmth when the sun is shining. And by putting that collector behind glass, much like in a green house or the inside of our cars, the warmth from the sun will be captured.

The Radiant Solar system uses a tube of glass rather than a flat plate or panel. Inside this tube is another glass tube and both are formed together to create a vacuum seal. This is known as an evacuated tube. The inner surface of this vacuum tube has a special dark coating to absorb greater amounts of solar warmth.

A temperature sensor is fitted to the collector to measure that temperature at all times. Another temperature sensor is fitted in the hot water tank to measure this temperature at all times. If the collector temperature is higher than the tank temperature the solar controller turns on the solar pump. If the collector temperature is not greater than the tank temperature then the pump will stop.

The controller makes sure that heat is never pumped out of the tank but only into the tank. The controller enables you to set the maximum tank temperature require for optimum efficiency.

A typical Solar water heating system uses a roof mounted collector (panel), angled towards the south to capture heat from the sun. About 1m2 of collector area is needed for each person in the household, with a minimum size of around 2.5m2. The collector contains a special solar fluid (usually a mixture of water and non-environmentally damaging anti-freeze) which is warmed by the sun's rays, and then pumped through a heat exchange coil in the hot water tank. As this coil warms up it transfers the heat to the water in the tank. The cooled solar fluid is then pumped back up to the collector to be reheated once again. An expansion vessel built into the system allows for any expansion or contraction of the solar fluid as it warms and cools.

Any solar-based system will still need a conventional water heater - powered by gas, oil or electricity - to provide back up. Remember, the sun doesn’t always shine! However, in hot weather solar energy should be sufficient to provide all the hot water needed, and even on a cool day it will help to raise the temperature of the water a little, meaning the conventional system doesn't have to work as hard.

solar heating diagram

In some cases the conventional water heater is used to heat a main water tank, with the solar heating system warming water in a second tank which feeds into the main supply via a diverter valve. It is simpler and more efficient, however, to have two heating coils in a single tank. One is heated by solar energy, and the other by the conventional heating source. The conventional heater takes over when a thermostat in the tank warns that the water temperature has fallen.

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Can I fit a solar water heating system to my existing hot water tank?

There is a special device we supply called a solar syphon that enables a new solar system to connect to an existing tank. This can be used to connect to either an open vented tank or an unvented tank and saves on the extra expense of a new tank. This device can be supplied with your Radiant Solar system when required.

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Which is best, flat panel or evacuated tube?

As far as performance is concerned the evacuated tube offers the best overall performance. The Radiant Solar system uses a special type of evacuated tube known as the ‘U’ tube. This means that there is a ‘U’ shaped tube inside the glass evacuated tube.

u pipe diagram

u pipe

The heating fluid flows down the ‘U’ tube inside each of the evacuated tubes collecting energy absorbed through the tube. There is a heat absorption aluminium heat transfer sheet to further assist the energy absorption. Behind the tubes is a special parabolic mirror to reflect and concentrate the suns energy all around the circumference of the tube. This ensures a fantastic rate of energy production compared to the simple flat panel design. The rate of heat absorption is greater than any other type of evacuated tube or flat panel system.

u pipe illustration

cpc photograph

However, for those of you who prefer the style and appearance of a flat panel, these can be substituted upon request.

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What happens when I don’t need any more hot water and the sun is shining?

When there is no further demand the heating fluid stops and stagnates in the collector. Because of the design of the system components the collector can tolerate temperatures up to 295 degrees C. The solar controller in our Radiant Solar system will periodically pump a small amount of this very hot fluid out of the collector to help protect the entire solar system and to continually monitor the situation.

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Solar for Swimming Pools

There are two ways of heating a swimming pool with solar warmth. The first is a direct type whereby the pool water is pumped through special solar collectors. These collectors are mostly just coils of plastic pipe mounted on a nearby roof. They perform particularly well in hot climates but are not best suited to the UK climate. Radiant do not supply this type of solar equipment.

The second and superior way is to use a standard solar collector and through a heat exchanger indirectly heat the pool water.

The Radiant Solar collector can be used to solely heat the pool or as part of some system integration schemes.

two floor house system diagram

When used as an addition to heating the hot water cylinder a special pool heat exchanger is used. This fits in to the pool filtration circuit in the same way as any pool heat exchanger does. Using the Radiant advanced solar controller surplus solar warmth is diverted into the special heat exchanger. The system design has to be such that large collector areas do not overpower the hot water cylinder or any other part of the system. It is advisable to use the Radiant system design service when considering heating your pool with solar warmth.

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Advantages of the Radiant Solar system
  1. Less panels (collectors) required than for standard flat panel systems
  2. Better year round performance, ideal for indoor pools and spa’s
  3. Can be integrated into the main heating system
  4. Can supplement the existing pool heating
  5. Good all year round performance thanks to the CPC technology

An alternative to solar is a dedicated swimming pool air source heat pump. For information on these see our air source Pool heat pump ASHP page.

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Technical Information on the Radiant Solar collector

The Radiant Solar collectors are manufactured by Ritter Solar, a leading name in the world of solar heating.

The different widths of the collector modules offer considerable flexibility in the design of tailor-made collector arrays. Short assembly times are guaranteed thanks to off-the-shelf collector units and simple technology for connecting several collectors in series. The use of high quality materials guarantees considerable operational security and a long service life. Thanks to the CPC reflector, it is ensured that each individual tube is always optimally oriented to the sun.

The CPC evacuated solar collectors provide high energy yield and long life expectancy. Produced from quality materials and components, the solar collectors are corrosion resistant and easy to install and maintain. Hailstone impact tests, in accordance to EN12975-2 and thermal shock tests by ITW ranks this collector amongst the finest solar system in the market place.

The CPC evacuated solar collectors are the most advanced technology solar collector available today. Designed specifically to perform efficiently in climates similar to the UK, the vacuum insulation and the CPC reflector produces a remarkable “all weather, all year round” performance. Manufactured from borosilicate glass 3.3 the solar tube is a product with improved geometry and performance. The evacuated tunnel consists of two concentric glass tubes which are sealed in a semicircular shape on one side and are joined to one another on the other side. The space between the tubes is evacuated and then hermetically sealed. The internal glass tube is coated with an environmentally friendly, highly ‘sensitive’ layer on the outside, thus turning it into an absorber. The coating is protected against adverse weathering influences within the evacuated space. The aluminium spatter coating used is characterised by extremely low emission and excellent absorption.

To increase the performance efficiency of evacuated tube collectors, a highly reflective, weather-proof CPC reflector (Compound Parabolic Concentrator) is fitted behind the evacuated tubes. The special, improved geometry of the reflector ensures that direct and diffused solar radiation falls on the absorber even when the angle of incidence is not ideal. This considerably improves the energy yield of the solar collector. A series of evacuated solar tubes fit into an insulated manifold chamber. The manifold heat exchanger and the heat transfer tubing inside the solar tubes are copper for maximum durability and performance. The flow and return pipes to the collector are fitted on the left or right of the manifold. Sensor pockets are provided on the manifold for simple and effective controller operation. The reflector is produced from a metal sheet with protective coating using accurate roll forming technology. Degradation over the life of the solar collector is minimal. Replacement of the reflector is easy because the special fastening technique allows the reflector to be replaced without using tools.

All Glass evacuated tubes (see figure 1 below) are the key component of the Radiant Solar Collectors and Solar Water Heaters.

Each evacuated tube consists of two glass tubes. The outer tube is made of extremely strong transparent borosilicate glass that is able to resist impact from hail up to 25mm in diameter. The inner tube is also made of borosilicate glass, but coated with a special selective coating, which features excellent solar heat absorption and minimal heat reflection properties.

The air is evacuated from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. This is why the tubes are able to absorb the energy from infrared rays which can pass through clouds. Wind and low temperatures also have less of effect on the function of evacuated tubes when compared to flat plate solar collectors due to the insulating properties of the vacuum.

ALL Glass evacuated tubes are aligned in parallel, the angle of mounting depends upon the latitude of your location. In a North South orientation the tubes can passively track heat from the sun all day. In an East West orientation they can track the sun all year round. The shape of the tubes provides superior absorption when compared to flat plate collectors for a number of reasons: 1) As the tube is round, the sun's rays are always striking the tubes surface at right angles, thus minimizing reflection.

If the collector surface is flat, the amount of solar radiation striking the collector surface is only at its maximum at midday when the sun is directly above the collector. In the morning or afternoon the sun's rays strike the collector's surface at an angle, and thus the amount of solar radiation that the collector is exposed to is reduced. Evacuated tubes, however, are round, and thus the amount of solar radiation striking the collector is relatively constant from mid morning to mid afternoon. This feature maximizes the total amount of solar radiation the collector is exposed to each day. Furthermore, the sun is always striking the tubes at an angle which is perpendicular to their surface thus reducing reflection. Any sunlight that is reflected off the glass surface or that passes between the tubes is reflected back to the underside of the tube by the reflective backing panel. The tubes exposure to, and subsequent absorption of solar radiation is therefore maximized. See figure 2 below.


structure glass solar collector tubestructure glass solar collector tube diagram


Solar Selective Absorption Coating (SURFACE)
A special coating that is of high absorbance and low emittance of solar radiation on surface of the absorber (Inner Tube)

Absorber of an All-Glass Evacuated Solar Collector Tube (here after as ABSORBER)
The inner glass tube with SOLAR SELECTIVE ABSORBING COATING on the surface. It transforms solar radiation into heat.

Vacuum gap in All-Glass Evacuated Tube
The vacuum space between inner and outer glass tube prevents heat losing from the tube by conduction and convection

Flash Getter
A kind of getter absorbs air when it is evaporated and concreted on the surface of inner tube.

Stagnation Temperature of an All-Glass Evacuated Tube
The immobile air in the all glass evacuated tube will reach its Max temperature when it is radiated under the designated solar radiation.

Stagnation Parameter of an All-Glass Evacuated Tube
Stagnation parameter = (Stagnation temperature - Environmental temperature)/Solar radiant intensity

Average Heat Loss Coefficient of All-Glass Evacuated Tube
There is heat loss and temperature decline of absorber if there is no solar radiance. AVERAGE HEAT LOSS COEFFICIENT is the power loss per area of absorber under the condition there is 1°C difference between the average water temperature inside the tube and the environmental temperature.

Evacuated tube collector CPC06/12/18 for domestic water heating and backup space heating, series connection Heat exchanger material: copper.




CPC 12

CPC 18

Number of evacuated tubes





η0 (Aperture area),
DIN 4757-4 or EN 12975





c1 with wind, in relation to aperture





c2 with wind, in relation to aperture





Kθ,trans (50°), in relation to aperture





Kθ,long (50°), in relation to aperture





Grid dimensions (length, height, depth)


0.70 x 1.64 x 0.1

1.39 x 1.64 x 0.1

2.08 x 1.64 x 0.1

Gross surface area





Aperture area





Collector contents










Max. working overpressure





Max. stagnation temperature





Connection diameter, clamping ring





Sensor sleeve





Collector material


Al / Cu / glass / silicone / PBT / EPDM / TE

Glass tube material


borosilicate glass 3.3

Selective absorber coating material


aluminium nitrite

Glass tube (Ø ext./Ø int./
wall thickness/tube len.)



Colour (aluminium frame profile, anodised)


aluminium grey

Colour (plastic parts)



Thermal shock test

ITW test no.


Hailstone test according to DIN EN 12975-2

TÜV test no.


Other tests and approvals


EN 12975, RAL UZ 73, Solar Keymark ISO 9001

DIN CERTCO - Register number



Certification (The Solar Keymark)

What is a 'Keymark'? The Keymark is a third-party certification mark, demonstrating to users and consumers the compliance of products with the requirements of the relevant CEN/CENELEC Standard(s).

Some of the basic European Mark Scheme rules
Type testing is performed by a third-party testing laboratory.
Type testing is performed by a Manufacturers shall apply a quality system of at least the level of the EN-ISO 9002 standard (maybe with a transitional period, of a maximum of three years).
There will be periodic surveillance of the products.
Bodies engaged in certification, testing and inspection need to fulfil the requirements of the relevant EN 45000/17000 series standard, and are accredited for the scope of their activity.

Download copy of solar keymark certificate (70KB)

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The intention of the Solar Keymark is that it will open up the European market for solar thermal products. The goal is for the Keymark to replace all the existing different national or regional certifications. A solar thermal product will need only one test, according to the European standard, and this test will be valid throughout Europe.

The consequences will be as follows: As the products are tested to the same criteria all over EU, it will be easier to compare different products and give fairer competition

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A mixture of propylene glycol, water and corrosion inhibitors that is able to withstand extremes of temperatures. This fluid should be inspected periodically and replaced when necessary.